Synchronous vibration is common in a rotating system and is usually caused by a rotating mass imbalance. Imbalance frequently occurs when a rotor of the rotating system has a mass center that is not coincident with its geometric center. Another form of vibration is non-synchronous rotor whirl, which occurs when a radial deflection of the rotor results in a tangential force normal to the deflection. The magnitude of the tangential force increases with the deflection. The rotating system can become unstable if there is insufficient damping of the aforementioned vibrations.
Both of these types of vibration are undesirable in a gas turbine engine and other types of turbo-machinery. To control these types of vibrations, a properly designed rotor support system that provides sufficient radial stiffness and damping are employed. The rotor support system may include bearings and a rotor support used to support the rotating rotor to a static housing, and a damper.
Various types of bearing can be used, including rolling element bearings. Some bearings may provide a radial interface between the rotor and the static structure, which can result in the bearing carrying only a radial load. Other bearings may additionally provide an axial interface between the rotor and the static structure, resulting in the bearing carrying a radial load and an axial load. To tailor stiffness of a rotor support system supported by rolling element bearings, the bearing(s) may be mounted to a bearing support. The bearing support may be designed to have uniform radial stiffness (i.e., isotropic stiffness) or non-uniform radial stiffness (i.e., anisotropic stiffness). For a rolling element bearing that reacts to axial loads, the bearing support may also have axial stiffness.
The rotor support is typically disposed between the bearings supporting the rotor and the static housing and may be used to compensate for system eccentricities. The rotor support generally includes a forward ring, an aft ring, and a plurality of beams extending therebetween. In one conventional configuration, the beams are each identically formed, thin, hour glass-shaped, and evenly spaced circumferentially around the rings, resulting in isotropic radial stiffness of the rotor. This type of rotor support has a circumferential axial stiffness. In another conventional configuration, the beams are I-shaped and divided into two groups. In this configuration, a first group is disposed on one side of the rings and a second group is located radially opposite from the first group providing anisotropic radial stiffness. This type of rotor support has a non-uniform circumferential axial stiffness.
Dampers of various forms can be placed between the rotor support and the static housing. The damper can be designed to provide the desired damping characteristics to complement the radial stiffness characteristics of the bearing support.
Although these conventional rotor support system configurations operate adequately, they still suffer from drawbacks. In particular, the rotor support and damper configuration including evenly spaced, identically formed, I-shaped beams may not provide sufficient radial stiffness, if rotor whirl occurs. And even though the rotor support including two groups of spaced apart beams improves radial stiffness to increase damping of rotor whirl, it may not provide sufficient circumferential axial stiffness, if the rotor experiences an axial thrust force thereon.
Hence, there is a need for a rotor support that has improved uniform circumferential axial stiffness capabilities over conventional rotor supports while providing radial stiffness and damping characteristics of the conventional configurations. In particular, it is desirable for the rotor support to provide suitable circumferential axial stiffness and to provide suitable radial stiffness and damping to dampen forces that may be exerted on the rotor support forward ring. Additionally, it is desirable for the rotor support and damper to be relatively inexpensive and simple to manufacture. Moreover, it is desirable for the rotor support and damper to be capable of being retrofitted into existing rotating machinery.